.background{

background-color: #008000;

height: 800px;

width: 420px;

margin: 0 auto;

}
.a1{

height: 80px;

width: 200px;

background: #fff;

margin-left: 50px;

margin-top: 10px;

margin-right: 10px;

float: left;
}
.a2{height: 80px;

  width: 100px;

  background-color: #fff;

  margin-top: 10px;

  float: left;

  }

.a3{

height: 80px;

width: 310px;

background-color: #FFFFFF;

margin-left: 50px;

margin-top: 10px;

float: left;

}

.a4{

height: 80px;

width: 310px;

background-color: #FFFFFF;

margin-left: 50px;

float: left;

margin-top: 10px;
}